In a hydraulic brake booster, a spool valve, slidably mounted in a housing, is biased to a brake-released position. A land on the spool valve cooperates with a corresponding land on the housing to substantially seal an inlet from a pressure chamber so that pressurized fluid is communicated to a steering gear. Since the land is contoured to regulate communication from the inlet to the pressure chamber as the spool valve moves away from the brake-released position, the land cannot readily be sealed for preventing seepage from the inlet to an exhaust passage or to the pressure chamber. Consequently, seepage past the land carries particles carried by the fluid into contact with the land. As the land is in close sliding engagement with a corresponding land on the housing, large particles carried by the seepage tend to wedge themselves between the lands so that upon operator actuation of the spool valve, the spool valve tends to stick to the housing to inhibit normal operation of the valve, when the fluid is overly contaminated.